Conventionally, there has been known a current integration method as one example of a method for estimating, at an optional time, an SOC of an energy storage device such as a secondary battery that is used while repeating charge and discharge. This is a method of measuring a watt-hour that is inputted/outputted with respect to a battery by measuring a charge-discharge current of the battery all the time, and adding or subtracting this charge-discharge current with respect to an initial available capacity in order to estimate the SOC. This method has an advantage that the SOC can be estimated during use of the battery. However, it has a disadvantage that a measurement error of a current sensor or the like is accumulated, so that measurement gradually becomes inaccurate, because the current is always measured to integrate a charge-discharge watt-hour.
Consequently, another method called an OCV method has been developed, in which an SOC estimation based on an open circuit voltage (OCV) of the battery is used in combination. There is a relatively accurate correlation between the OCV and SOC when a current is not flowing in/from the battery. Thus, a battery voltage when the current is not flowing in/from the battery (open circuit voltage) is measured in order to find the SOC corresponding to the measured OCV with reference to the correlation between the OCV and the SOC stored in advance, and to correct the SOC estimated by the current integration method. This allows the accumulation of error to be cut off, which can increase accuracy of the SOC estimation by the current integration method.
In recent years, a lithium ion battery using iron lithium phosphate as a positive active material has been gaining attention. It has been known that in this type of lithium ion battery, there exists in the OCV-SOC characteristics a flat (plateau) region where the OCV hardly changes while the SOC changes in a wide range as shown in FIG. 1, for example. In this type of lithium ion battery, correction of the error caused in the SOC estimation becomes difficult even by the OCV method.
In the case of a lithium ion battery having the OCV-SOC characteristics as in FIG. 1, if the OCV is, for example, about 3.33 V, which means that the OCV is in the voltage flat region, it can be only said that the SOC is any of 15% to 95%. Thus, in this type of battery, the OCV reset can be only conducted in a voltage inclination region where the OCV exhibits a certain degree of inclination in the OCV-SOC characteristics. Thus, the frequency of performing the OCV reset becomes lower, so that there has been a limit in the accuracy improvement of the SOC estimation.
The above-mentioned error of the SOC estimation may bring about an unfavorable situation of power deficiency, particularly in an electric car that uses a battery as a driving source. Thus, a solution to this problem is desired intensely.
In a technique disclosed in JP2010-266221 A, when it is detected that the SOC has changed from a value lower from a lower limit value in the voltage flat region into the voltage flat region by charge, the SOC is reset to the lower limit in the voltage flat region.
In the technique disclosed in JP2010-266221 A, after the battery is considerably discharged, the timing that the SOC changes from the value lower than the lower limit value of the voltage flat region into the voltage flat region is grasped. Thus, the frequency of performing the OCV reset is not necessarily high, so that there is a limit in the accuracy improvement.